1. Field of the Invention
The invention is directed to a hand-held pulse laser distance measuring device, in particular a hand-held construction laser distance measuring device.
2. Description of the Prior Art
In the building industry, distances must be exactly determined with an accuracy of within a few mm at a range of up to several hundreds of meters distance. The hand-held laser distance measuring devices which are suitably constructed for this purpose and to which the present invention is directed use a pulse measurement method with a modulated visible laser beam for measuring distance. Artificial light sources operating at 50 Hz AC oscillate in intensity with a frequency fINT of 100 Hz and at 60 Hz AC with a frequency fINT of 120 Hz. This frequency doubling is brought about because the intensity maximum occurs at both the positive maximum and negative maximum of the alternating current. This alternating component of the frequency fINT is detected, along with the measurement light pulses and reference light pulses, as an interference signal by the photodiode and is sampled by the analog-to-digital converter. It leads to measurement errors in distance measurement.
DE3103567C2 introduces a method for direct measurement of the light pulse time-of-flight in which a measurement light pulse traveling over the measurement distance and a reference light pulse traveling over the reference distance are detected by the same photodetector. The detected measurement light pulse and reference light pulse start and stop a time measurement system, e.g., a fast counter. The measurement distance is determined definitively by direct and definitive measurement of the time difference between the detection of the reference light pulse and the detection of the measurement light pulse.
According to DE3429062C2, a device of the type mentioned above for measuring the time of flight of light waves has a superimposition module in which triggered individual measurements and measurement light pulses and reference light pulses are periodically superimposed and accordingly accumulated.
DE 10112833C1 discloses a hand-held laser distance measuring device with a pulse reflection mixing method. The detection pulse train detected by the light detector or, in case of separate light detectors, the reference pulse train on the one hand and the measurement pulse train on the other hand are preferably directly subjected to direct mixing in the respective light detector followed by low-pass filtering. The direct mixing is controlled by a LO pulse train which is locally generated at the measurement point and whose duty factor is equal to, or approximately equal to, the duty factor of the measurement pulse train and whose repetition frequencies are selected so as to differ slightly. Accordingly, the mixing pulse repetition frequency fMIX of the low-frequency pulse train corresponds to the amount of the difference between the pulse repetition frequency f of the transmission pulse train and measurement pulse train on the one hand and the pulse repetition frequency of the local oscillator pulse train fLO on the other hand. Therefore: fMIX=|f−fLO|. Like the high-frequency detection pulse train, the low-frequency pulse train likewise comprises reference pulses and measurement pulses whose time delay is a measure of the distance. For further particulars, the person skilled in the art is referred to the above-cited document, whose disclosure is explicitly incorporated herein in its entirety.
It is the object of the invention to realize a hand-held laser distance measuring device with pulse reflection mixing having enhanced protection against interference.